diff --git a/bin/project b/bin/project
index 4a23a8f..14cb21b 100755
--- a/bin/project
+++ b/bin/project
@@ -26,6 +26,8 @@ module Bin
     pre do |global_options, _command, _options, _args|
       GitHub::Progress.enabled = global_options['quiet'] ? false : true
 
+      GitHub::Data.check_dups!
+
       if global_options['vcr-cassette-name']
         require 'webmock'
         WebMock.enable!
diff --git a/lib/github/data.rb b/lib/github/data.rb
index 794eca2..a6ac322 100644
--- a/lib/github/data.rb
+++ b/lib/github/data.rb
@@ -46,6 +46,14 @@ def students
         @students ||= load_list(STUDENTS)
       end
 
+      def check_dups!
+        %i[members contractors students external_users].combination(2).each do |l, r|
+          send(l).intersection(send(r)).each do |user|
+            warn "WARNING: #{user} is found in both #{l} and #{r}"
+          end
+        end
+      end
+
       def bots
         @bots ||= load_list(BOTS)
       end